As bandwidth continues to grow, capacity is being stretched in fiber optic networks (Wavelength Division Multiplexing (WDM), optical line systems, etc.). Various advanced techniques are used to provide additional bandwidth including advanced coherent modulation techniques, polarization multiplexing, flexible grid spacing, and the like in optical line systems. With these advanced techniques, optical line systems suffer new degrees of susceptibility to transients on a time varying optical channel, including polarization transients, etc. As described herein, an State of Polarization (SOP) transient source causes an SOP transient which is anything that causes quick changes in the SOP of signals on an optical fiber. Coherent optical line systems are configured to track SOP changes as part of normal operation, but when quick SOP transients occur, these can be outside the tracking ability, leading to errors, loss of framing, etc. Example SOP transient sources can include, without limitation, external vibrations (e.g., bridges, roads, railroads, wind, etc.), bad splices on the optical fiber, weather (e.g., lightning, wind, etc.), or the like. Usually, SOP transient sources behave unpredictably and from unknown locations in the fiber. Further, SOP transient sources can be time specific, seasonal, etc. Thus, it is important in optical line systems to have some technique for real-time, in-service measurement. SOP transients can be particularly problematic with coherent transmission, and especially difficult to localize/isolate given their nature in general. An application using a polarimeter with an optical line system for SOP transient localization is described in commonly-assigned U.S. patent application Ser. No. 14/865,802 filed Sep. 25, 2015, and entitled “SYSTEMS AND METHODS USING A POLARIMETER TO LOCALIZE STATE OF POLARIZATION TRANSIENTS ON OPTICAL FIBERS,” the contents of which are incorporated by reference.
Polarimeters are polarization measurement devices configured to measure the polarization state of an optical signal. Conventionally, in optical line systems (e.g., WDM, Dense WDM, etc.), polarimeters are external test devices which connect to the optical line system, i.e., polarimeters are not integrated devices. Accordingly, conventional polarimeters do not operate in-service, i.e., with traffic-bearing signals active on the optical line system. Further, conventional polarimeters rely on free space optics which is not amenable to integration with fiber components (i.e., non-free space optics). Conventional polarimeters require special design of bulk free space optics components and/or special coupling functions of an optical fiber leading to costly and bulky implementations. Finally, conventional polarimeters are cost prohibitive for optical line system applications as well as complex providing vast more information than necessary for such optical line system applications, i.e., only SOP tracking is required to monitor the health of an optical fiber.
It would be advantageous to provide a polarimeter integrated with an optical line system optimized to provide functionality necessary for monitoring the health of the optical line system.